This invention relates to an improved method of casting unitary, tubular, simipermeable polysulfone ultrafiltration membranes in place, in the interior feed bores of a support module made of bonded, resin coated filler particles, for use in ultrafiltration fluid purification systems.
Ultrafiltration is the name used to describe the process of filtering solute molecules at least one to two times larger than solvent molecules. Some large molecules can be filtered without developing significant osmotic pressure, and because of this, the operating pressures for ultrafiltration may be as low as 15 psi. Reverse osmosis, on the other hand, refers to the filtering of solutes of the same order of magnitude in size as the solvent, at minimum pressures of about 400 psi.
An ultrafiltration system has application in many areas. The areas of greatest present interest are making potable water from polluted water and cleaning up waste streams. Other applications include the concentration of food (orange juice, tomato juice, cheese whey, and syrups), and protein and paint solids removal.
Ultrafiltration fluid purification systems have taken on many forms, including plate and frame, hollow fine fiber, spiral wound and tubular types. The configuration which appears to have the greatest commercial appeal at the present time is the tubular type system. The typical tubular type system generally employs a simipermeable cellulose acetate membrane in the form of a long, straight tube containing an active osmotic skin. This cellulose acetate membrane can be drop cast, in situ, inside modules made of bonded, resin coated filler particles, as taught by Stana et al in U.S. Pat. No. 3,657,402.
The main difficulty with cellulose acetate membranes, however, is the preparation of a membrane strong enough to withstand the prolonged passage of high water flows without compacting; while still retarding the passage of selected ions or molecules. It is also known that cellulose acetate membrane applications are limited by their swelling, due to organic solvents, by their thermal instability and by their degradation in high pH environments.
H. K. Lonsdale in "Reverse Osmosis Membrane Research", Plenum Press 1972, taught direct drop casting, into a steel casting tube, of thin, porous, high strength, chemically resistant, polysulfone support linings for polyacrylic acid, cellulose acetate and cellulose nitrate reverse osmosis membranes. Baker, in U.S. Pat. No. 3,567,810, formed flat polysulfone reverse osmosis membrane films, by forced evaporation of solvent from the film in an oven at 50.degree.-250.degree. C. These films were formed on a glass substrate from 14 wt.% polysulfone, dissolved at 50.degree.-80.degree. C. in combined acetone-dimethylsulfoxide solvent. Quentin, in U.S. Pat. No. 3,709,841, taught placing flat or tubular partially sulfonated polysulfone ion exchange membranes in water purification systems. The polysulfone was first partially sulfonated, wherein sulphonic radicals were substituted for benzene rings, by reaction with chlorosulphonic acid in 1,2 dichloroethane. The membrane was cast on a glass substrate from 3-13 wt.% sulphonated polysulfone dissolved in dimethylformamide at ambient temperature, and solvent force evaporated in an oven at 50.degree. C.
The low solids polysulfone casting solution formed by heat dissolution, and the low solids sulfonated polysulfone casting solution formed by dissolution at ambient conditions, are unstable, tending to separate easily. They are not particularly suitable for drop casting into porous support modules having walls made of bonded, resin coated filler particles. In this type module, a particularly homogeneous, highly viscous composition, able to easily wet, penetrate, conform and adhere to the relatively rough casting surface is required.